1. Field of the Invention
The present invention relates to a toroidal-type continuously variable transmission (CVT) for automobiles, which is used to continuously control change gear ratio, and more specifically to a surface roughness structure of rolling elements of the toroidal-type CVT, such as an input disk, an output disk and a power roller.
U.S. Pat. No. 5,676,618 discloses one example of the toroidal-type CVT, which is incorporated herein by reference.
FIG. 1 shows the basic structure of the toroidal-type CVT. The toroidal-type CVT includes a plurality of metal rolling elements contacting one another through a traction oil film. The rolling elements include input disk 3 connected with input shaft 1, output disk 5 connected with output shaft 2, and power rollers 6, 6 interposed between input disk 3 and output disk 5 and rotatable to transmit rotational force from input disk 3 to output disk 5. Each power roller 6 has a tiltable roller shaft such that power roller 6 is inclined relative to input and output disks 3 and 5 when the roller shaft tilts. Power roller 6 is contacted with input disk 3 and output disk 5 through a traction oil. When power roller 6 is inclined, the contact between power roller 6 and input and output disks 3 and 5 shifts. This changes the ratio of the torque radius of input disk 3 to that of output disk 5 to thereby continuously change the transmission ratio.
Table 1 shows one example of the results of measurement of a surface structure or texture, specifically, a surface roughness, of the mutually contact surfaces of input and output disks 3 and 5 and power roller 6 of the toroidal-type CVT, which surfaces are hereinafter referred to as traction surfaces.
Generally, the traction surfaces of the rolling elements of the toroidal-type CVT in the earlier technique have the surface structure in which arithmetical mean roughness Ra prescribed by JIS B0601-1994 is not more than 0.05 xcexcm, root-mean-square roughness Rq is not more than 0.07 xcexcm, oil retention volume Vo is not more than 1.3xc3x9710xe2x88x925 mm3/mm2, and oil retention depth ratio K is less than 0.9.
If the surface roughness of the traction surfaces of the rolling elements exceeds a certain value relative to a thickness of the traction oil film formed between input and output disks 3 and 5 and power roller 6, rolling-fatigue lives of input and output disks 3 and 5 and power roller 6 are deteriorated so that durability of the CVT decreases. Therefore, the traction surfaces are subjected to grinding and super-finishing such that the surface roughness is limited to a sufficiently small level in height, that is, arithmetical mean roughness Ra of not more than 0.05 xcexcm. Here, as prescribed in JIS B 0601, arithmetical mean roughness Ra is determined as the value obtained by the following formula and expressed in micrometer (xcexcm) when sampling only the reference length L from the roughness curve in the direction of mean line, taking X-axis in the direction of mean line and Y-axis in the direction of longitudinal magnification of this sampled part and the roughness curve is expressed by y=f(x):                     Ra        =                              1            L                    ⁢                                    ∫              0              L                        ⁢                                          "LeftBracketingBar"                                  f                  ⁡                                      (                    x                    )                                                  "RightBracketingBar"                            ⁢                              ⅆ                                  (                  x                  )                                                                                        (        1        )            
where L is reference length.
Namely, arithmetical mean roughness Ra means the mean deviation obtained by dividing the area defined by the roughness curve f(x) and the mean line, i.e., X-axis, as shown in FIG. 2A, by the reference length L.
Root-mean-square roughness Rq is determined as the value obtained by the following formula and expressed in micrometer (xcexcm) when sampling only the reference length L from the roughness curve in the direction of mean line, taking X-axis in the direction of mean line and Z-axis in the direction of longitudinal magnification of this sampled part and the roughness curve is expressed by z=f(x):                     Rq        =                                            1              L                        ⁢                                          ∫                0                L                            ⁢                                                                    f                    2                                    ⁡                                      (                    x                    )                                                  ⁢                                  ⅆ                                      (                    x                    )                                                                                                          (        2        )            
where L is reference length.
Namely, root-mean-square roughness Rq means the square root of the mean deviation obtained by dividing the area defined by the mean line (X-axis) and the curve obtained by squaring the distance between the roughness curve f(x) and the mean line (X-axis), as shown in FIG. 2B, by the reference length L.
DIN4776 defines parameters Mr1, Mr2, Rpk, Rvk and Rk for evaluation of lubricating characteristic of a surface structure, based on an initial wear part, a substantial contact part, and an oil retention part, which are separated from a bearing curve (material ratio curve). Parameters Mr1, Mr2, Rpk, Rvk and Rk are determined as follows.
(1) Mr1: Material Portion 1
Level, in percent, determined for the intersection line which separates peaks from roughness profile and cooperates with Mr2 described later to determine the roughness core profile which is roughness profile excluding the peaks and deep valleys (see FIG. 3). Mr1 is calculated as follows. As shown in the right part of FIG. 3, slope line SLsg includes the secant line of material ratio curve MrC over 40% of the material ratio which shows the smallest gradient. This is determined by moving the secant line for xcex94Mr=40% along material ratio curve MrC. Intersection of a lower limit line at Mr=0% and slope line SLsg with the smallest gradient is indicated at a. Intersection of material ratio curve MrC and a horizontal line passing through intersection a is indicated at c. Material ratio at intersection c is expressed by Mr1 (%). Mr1 indicates the material portion after initial wear.
(2) Mr2: Material Portion 2
Level, in percent, determined for the intersection line which separates deep valleys from the roughness profile (see FIG. 3). Mr2 is calculated as follows. As illustrated in FIG. 3, intersection of an upper limit line at Mr=100% and slope line SLsg with the smallest gradient is indicated at b. Intersection of material ratio curve MrC and a horizontal line passing through intersection b is indicated at d. Material ratio at intersection d is represented by Mr2 (%). Mr2 indicates the material portion after long-period wear.
(3) Rpk: Reduced Peak Height
Average height of the peaks above the roughness core profile. In FIG. 3, if an area of a right triangle formed by base ac and a side lying on the lower limit line Mr=0% is equal to an area defined by the lower limit line Mr=0%, base ac and material ratio curve MrC, the height of the right triangle is expressed as Rpk (xcexcm). In other words, the distance between intersection a and a vertex of the right triangle which is located on the lower limit line Mr=0% is represented by Rpk (xcexcm). Rpk indicates a height of initial wear.
(4) Rvk: Reduced Valley Depth
Average depth of the profile valleys projecting through the roughness core profile. In FIG. 3, if an area of a right triangle formed by base bd and a side lying on the upper limit line Mr=100% is equal to an area defined by the upper limit line Mr=100%, base bd and material ratio curve MrC, the height of the right triangle, namely, the distance between intersection b and a vertex of the right triangle which is located on the upper limit line Mr=100%, is represented by Rvk (xcexcm). Rvk indicates a depth of oil retention valley.
(5) Rk: Core Roughness Depth
Height difference between intersections c and d is represented by Rk (xcexcm). Rk indicates a height of long-period wear which is reduced by wear during a long period until the surface is worn out to unuseable state.
Vo and K are determined as follows.
Vo: oil retention volume
Vo is represented by the following formula:
Vo=[(100xe2x88x92Mr2)xc3x97Rvk]/200000(mm3/mm2)xe2x80x83xe2x80x83(3)
Vo indicates a volume of oil retained in oil retention depth Rvk per 1 mm2 (see FIG. 4).
K: oil retention depth ratio
K is a ratio of oil retention depth Rvk to core roughness depth Rk and represented by the following formula:
K=Rvk/Rk(dimensionless number)xe2x80x83xe2x80x83(4)
As oil retention depth ratio K increases, lubrication characteristic of the surface becomes better.
Automobiles are required to operate under various environmental conditions in which the CVTs tend to operate with a traction oil having the remarkably wide temperature range from approx. xe2x88x9230xc2x0 C. to approx. 120xc2x0 C. Since the traction oil must maintain a good fluidity at the extremely low temperature, the viscosity of the traction oil at high temperature becomes much lower. Therefore, if the surface roughness of the traction surfaces of input and output disks 3 and 5 and power roller 6 of the above-described CVT is sufficiently small with respect to the thickness of the traction oil film formed therebetween, a force transmittable between the traction surfaces relative to a pressing force applied to the traction surfaces decreases along with the temperature rise at the traction surfaces. For instance, if surface roughness Ra is 0.5 xcexcm or less as explained in the above-described earlier technique with respect to the traction oil film having the thickness of about 0.2 xcexcm, the transmission force relative to the pressing force becomes small as the temperature at the traction surfaces increases. In order to transmit a driving force regardless temperature conditions upon operation of the CVTs, it is necessary to apply such a large pressing force as to produce a sufficiently large transmission force between the traction surfaces even at high temperature.
The ratio of the transmission force to the pressing force is traction coefficient. Accordingly, if the traction coefficient is small, a large pressing force will be required to be applied to the rolling elements for obtaining a predetermined transmission force. Then, respective components will be increased in weight in order to assure the strength of the components against the large pressing force. Further, friction loss of bearings supporting the components will increase, leading to loss of automobile power. There is a demand for eliminating such undesired possibilities in the CVT in the earlier technique.
An object of the present invention is to provide a toroidal-type continuously variable transmission (CVT) for automobiles which includes rolling elements capable of maintaining high traction coefficient even upon high-temperature operation while keeping the formation of a traction oil film as carried out in the toroidal-type CVT in the earlier technique, and capable of transmitting a large driving force without increasing the pressing force to be applied to the rolling elements of the toroidal-type CVT.
In the toroidal-type CVT of the present invention, the rolling elements have the traction surfaces having a surface microstructure defined with respect to a thickness of the traction oil film formed under the operating condition that the temperature of a traction portion of each traction surface is relatively high. The toroidal-type CVT of the present invention, therefore, can serve for maintaining rolling-fatigue lives of the rolling elements as kept in the rolling elements of the toroidal-type CVT in the earlier technique. The toroidal-type CVT of the present invention also can serve for reducing the pressing force, so that the components of the CVT can be prevented from the increase in weight that is caused due to increase in the pressing force. Further, the toroidal-type CVT of the present invention can contribute to suppression of friction loss at bearings supporting the components to thereby reduce loss of automobile power.
According to one aspect of the present invention, there is provided a toroidal-type continuously variable transmission for automobiles, comprising:
a plurality of rolling elements having traction surfaces cooperating with each other to transmit a power between the rolling elements via a traction oil film formed between the traction surfaces,
wherein a ratio h/Rqsyn is not more than 3.0 when an oil retention depth ratio K of at least one of the traction surfaces is not less than 0.9, and an oil retention volume Vo of the at least one of the traction surfaces is not less than 7xc3x9710xe2x88x926 mm3/mm2,
where
h is a thickness of the traction oil film formed under the operating condition, and
Rqsyn is a root-sum-square value of root-mean-square roughness values Rq of the traction surfaces. Ratio h/Rqsyn is preferably not more than 1.0. Ratio h/Rqsyn is more preferably in a range of 0.2-1.0. Thickness h of the traction oil film is calculated on the basis of dimension and material characteristics of the traction surfaces, temperature condition, operating condition of the continuously variable transmission, and viscosity characteristics of the traction oil according to an elastohydrodynamic lubrication theory. Thickness h of the traction oil film can be calculated using the equation of Hamrock and Dowson:
H=3.42 gv0.49gE0.17(1xe2x88x92exe2x88x920.68 k)
where
H=(h/Rx)(W/U)
gv=GW3/U2 
gE=W8/3/U2 
k=(Ry/Rx)2/xcfx80
U=xcex7ou/(Exe2x80x2Rx)
W=w/(Exe2x80x2Rx2)
G=xcex1Exe2x80x2
where
H is a parameter of a film thickness, gv is a parameter of viscosity, gE is a parameter of elasticity, and k is a parameter of ellipse, Rx is an equivalent radius of curvature in a rolling direction of the traction portion of the traction surface, W is a parameter of load, U is a parameter of speed, G is a parameter of material, Ry is an equivalent radius of curvature in a direction perpendicular to the rolling direction of the traction portion of the traction surface, xcex7o is an oil viscosity under atmospheric pressure, u is a rolling speed of the traction portion of the traction surface, Exe2x80x2 is an equivalent vertical elastic coefficient of the traction portion of the traction surface, w is a pressing force applied to the traction portion of the traction surface, and xcex1 is a pressure viscosity coefficient. The operating condition may be the condition that an engine output is maximum and a temperature of a traction oil to be supplied to the traction surfaces is highest.
According to a further aspect of the present invention, there is provided a toroidal-type continuously variable transmission for automobiles, comprising:
a plurality of rolling elements having traction surfaces cooperating with each other to transmit a power between the rolling elements via a traction oil film formed between the traction surfaces,
wherein an oil retention depth ratio K of at least one of the traction surfaces is not less than 0.9 and a ratio h/Vosyn is not more than 15.0,
where
h is a thickness of the traction oil film formed under the operating condition, and
Vosyn is a root-sum-square value of oil retention volumes Vo of the traction surfaces. Ratio h/Vosyn is preferably not more than 5.0. Further, thickness h of the traction oil film can be calculated using an elastohydrodynamic lubrication theory on the basis of dimension and material characteristics of the traction surfaces, temperature condition, operating condition of the toroidal-type CVT, and viscosity characteristics of the traction oil according to an elastohydrodynamic lubrication theory. Thickness h of the traction oil film can be calculated using the equation of Hamrock and Dowson. The operating condition is the condition that an engine output is maximum and a temperature of the traction oil to be supplied to the traction surfaces is highest.
According to a still further aspect of the present invention, there is provided a toroidal-type continuously variable transmission for automobiles, comprising:
a plurality of rolling elements having traction surfaces cooperating with each other to transmit a power between the rolling elements via a traction oil film formed between the traction surfaces,
wherein an oil retention depth ratio K of at least one of the traction surfaces is not less than 0.9, an oil retention volume Vo of the at least one of the traction surfaces is not less than 7xc3x9710xe2x88x926 mm3/mm2, and a root-sum-square value Rqsyn of root-mean-square roughness values Rq of the traction surfaces is not less than 0.07 xcexcm. Root-sum-square value Rqsyn is preferably not less than 0.2 xcexcm. Root-sum-square value Rqsyn is more preferably in a range of 0.2-1.0 xcexcm from a viewpoint of durability.
According to a still further aspect of the present invention, there is provided a toroidal-type continuously variable transmission for automobiles, comprising:
a plurality of rolling elements having traction surfaces cooperating with each other to transmit a power between the rolling elements via a traction oil film formed between the traction surfaces,
wherein an oil retention depth ratio K of at least one of the traction surfaces is not less than 0.9 and a root-sum-square value Vosyn of oil retention volumes Vo of the traction surfaces is not less than 1.3xc3x9710xe2x88x925 mm3/mm2. Root-sum-square value Vosyn is preferably not less than 4xc3x9710xe2x88x925 mm3/mm2.